问题
I need to design tables which stores all the metadata of files (i.e., file name, author, title, date created), and custom metadata (which has been added to files by users, e.g. CustUseBy, CustSendBy). The number of custom metadata fields cannot be set beforehand. Indeed, the only way of determining what and how many custom tags have been added on files is to examine what exists in the tables.
To store this, I have created a base table (having all common metadata of files), an Attributes table (holding additional, optional attributes that may be set on files) and a FileAttributes table (which assigns a value to an attribute for a file).
CREAT TABLE FileBase (
id VARCHAR(32) PRIMARY KEY,
name VARCHAR(255) UNIQUE NOT NULL,
title VARCHAR(255),
author VARCHAR(255),
created DATETIME NOT NULL,
) Engine=InnoDB;
CREATE TABLE Attributes (
id VARCHAR(32) PRIMARY KEY,
name VARCHAR(255) NOT NULL,
type VARCHAR(255) NOT NULL
) Engine=InnoDB;
CREATE TABLE FileAttributes (
sNo INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
fileId VARCHAR(32) NOT NULL,
attributeId VARCHAR(32) NOT NULL,
attributeValue VARCHAR(255) NOT NULL,
FOREIGN KEY fileId REFERENCES FileBase (id),
FOREIGN KEY attributeId REFERENCES Attributes (id)
) Engine=InnoDB;
Sample data:
INSERT INTO FileBase
(id, title, author, name, created)
VALUES
(\'F001\', \'Dox\', \'vinay\', \'story.dox\', \'2009/01/02 15:04:05\'),
(\'F002\', \'Excel\', \'Ajay\', \'data.xls\', \'2009/02/03 01:02:03\');
INSERT INTO Attributes
(id, name, type)
VALUES
(\'A001\', \'CustomeAttt1\', \'Varchar(40)\'),
(\'A002\', \'CustomUseDate\', \'Datetime\');
INSERT INTO FileAttributes
(fileId, attributeId, attributeValue)
VALUES
(\'F001\', \'A001\', \'Akash\'),
(\'F001\', \'A002\', \'2009/03/02\');
Now the problem is I want to show the data in a manner like this:
FileId, Title, Author, CustomAttri1, CustomAttr2, ...
F001 Dox vinay Akash 2009/03/02 ...
F002 Excel Ajay
What query will generate this result?
回答1:
The question mentions MySQL, and in fact this DBMS has a special function for this kind of problem: GROUP_CONCAT(expr). Take a look in the MySQL reference manual on group-by-functions. The function was added in MySQL version 4.1. You'll be using GROUP BY FileID in the query.
I'm not really sure about how you want the result to look. If you want every attribute listed for every item (even if not set), it will be harder. However, this is my suggestion for how to do it:
SELECT bt.FileID, Title, Author,
GROUP_CONCAT(
CONCAT_WS(':', at.AttributeName, at.AttributeType, avt.AttributeValue)
ORDER BY at.AttributeName SEPARATOR ', ')
FROM BaseTable bt JOIN AttributeValueTable avt ON avt.FileID=bt.FileID
JOIN AttributeTable at ON avt.AttributeId=at.AttributeId
GROUP BY bt.FileID;
This gives you all attributes in the same order, which could be useful. The output will be like the following:
'F001', 'Dox', 'vinay', 'CustomAttr1:varchar(40):Akash, CustomUseDate:Datetime:2009/03/02'
This way you only need one single DB query, and the output is easy to parse. If you want to store the attributes as real Datetime etc. in the DB, you'd need to use dynamic SQL, but I'd stay clear from that and store the values in varchars.
回答2:
The general form of such a query would be
SELECT file.*,
attr1.value AS 'Attribute 1 Name',
attr2.value AS 'Attribute 2 Name',
...
FROM
file
LEFT JOIN attr AS attr1
ON(file.FileId=attr1.FileId and attr1.AttributeId=1)
LEFT JOIN attr AS attr2
ON(file.FileId=attr2.FileId and attr2.AttributeId=2)
...
So you need to dynamically build your query from the attributes you need. In php-ish pseudocode
$cols="file";
$joins="";
$rows=$db->GetAll("select * from Attributes");
foreach($rows as $idx=>$row)
{
$alias="attr{$idx}";
$cols.=", {$alias}.value as '".mysql_escape_string($row['AttributeName'])."'";
$joins.="LEFT JOIN attr as {$alias} on ".
"(file.FileId={$alias}.FileId and ".
"{$alias}.AttributeId={$row['AttributeId']}) ";
}
$pivotsql="select $cols from file $joins";
回答3:
If you're looking for something more usable (and joinable) than a group-concat result, try this solution below. I've created some tables very similar to your example to make this make sense.
This works when:
- You want a pure SQL solution (no code, no loops)
- You have a predictable set of attributes (e.g. not dynamic)
- You are OK updating the query when new attribute types need to be added
- You would prefer a result that can be JOINed to, UNIONed, or nested as a subselect
Table A (Files)
FileID, Title, Author, CreatedOn
Table B (Attributes)
AttrID, AttrName, AttrType [not sure how you use type...]
Table C (Files_Attributes)
FileID, AttrID, AttrValue
A traditional query would pull many redundant rows:
SELECT * FROM
Files F
LEFT JOIN Files_Attributes FA USING (FileID)
LEFT JOIN Attributes A USING (AttributeID);
AttrID FileID Title Author CreatedOn AttrValue AttrName AttrType 50 1 TestFile Joe 2011-01-01 true ReadOnly bool 60 1 TestFile Joe 2011-01-01 xls FileFormat text 70 1 TestFile Joe 2011-01-01 false Private bool 80 1 TestFile Joe 2011-01-01 2011-10-03 LastModified date 60 2 LongNovel Mary 2011-02-01 json FileFormat text 80 2 LongNovel Mary 2011-02-01 2011-10-04 LastModified date 70 2 LongNovel Mary 2011-02-01 true Private bool 50 2 LongNovel Mary 2011-02-01 true ReadOnly bool 50 3 ShortStory Susan 2011-03-01 false ReadOnly bool 60 3 ShortStory Susan 2011-03-01 ascii FileFormat text 70 3 ShortStory Susan 2011-03-01 false Private bool 80 3 ShortStory Susan 2011-03-01 2011-10-01 LastModified date 50 4 ProfitLoss Bill 2011-04-01 false ReadOnly bool 70 4 ProfitLoss Bill 2011-04-01 true Private bool 80 4 ProfitLoss Bill 2011-04-01 2011-10-02 LastModified date 60 4 ProfitLoss Bill 2011-04-01 text FileFormat text 50 5 MonthlyBudget George 2011-05-01 false ReadOnly bool 60 5 MonthlyBudget George 2011-05-01 binary FileFormat text 70 5 MonthlyBudget George 2011-05-01 false Private bool 80 5 MonthlyBudget George 2011-05-01 2011-10-20 LastModified date
This coalescing query (approach using MAX) can merge the rows:
SELECT
F.*,
MAX( IF(A.AttrName = 'ReadOnly', FA.AttrValue, NULL) ) as 'ReadOnly',
MAX( IF(A.AttrName = 'FileFormat', FA.AttrValue, NULL) ) as 'FileFormat',
MAX( IF(A.AttrName = 'Private', FA.AttrValue, NULL) ) as 'Private',
MAX( IF(A.AttrName = 'LastModified', FA.AttrValue, NULL) ) as 'LastModified'
FROM
Files F
LEFT JOIN Files_Attributes FA USING (FileID)
LEFT JOIN Attributes A USING (AttributeID)
GROUP BY
F.FileID;
FileID Title Author CreatedOn ReadOnly FileFormat Private LastModified 1 TestFile Joe 2011-01-01 true xls false 2011-10-03 2 LongNovel Mary 2011-02-01 true json true 2011-10-04 3 ShortStory Susan 2011-03-01 false ascii false 2011-10-01 4 ProfitLoss Bill 2011-04-01 false text true 2011-10-02 5 MonthlyBudget George 2011-05-01 false binary false 2011-10-20
回答4:
This is the standard "rows to columns" problem in SQL.
It is most easily done outside SQL.
In your application, do the following:
Define a simple class to contain the file, the system attributes, and a Collection of user attributes. A list is a good choice for this collection of customer attributes. Let's call this class FileDescription.
Execute a simple join between the file and all of the customer attributes for the file.
Write a loop to assemble FileDescriptions from the query result.
Fetch the first row, create a FileDescription and set the first customer attribute.
While there are more rows to fetch:
- Fetch a row
- If this row's file name does not match the FileDescription we're building: finish building a FileDescription; append this to a result Collection of File Descriptions; create a fresh, empty FileDescription with the given name and first customer attribute.
- If this row's file name matches the FileDescription we're building: append another customer attribute to the current FileDescription
回答5:
I have been experimenting with the different answers and Methai's answer was the most convenient for me. My current project, although it does uses Doctrine with MySQL, has quite a few loose tables.
The following is the result of my experience with Methai's solution:
create entity table
DROP TABLE IF EXISTS entity;
CREATE TABLE entity (
id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
title VARCHAR(255),
author VARCHAR(255),
createdOn DATETIME NOT NULL
) Engine = InnoDB;
create attribute table
DROP TABLE IF EXISTS attribute;
CREATE TABLE attribute (
id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
name VARCHAR(255) NOT NULL,
type VARCHAR(255) NOT NULL
) Engine = InnoDB;
create attributevalue table
DROP TABLE IF EXISTS attributevalue;
CREATE TABLE attributevalue (
id INT UNSIGNED PRIMARY KEY AUTO_INCREMENT,
value VARCHAR(255) NOT NULL,
attribute_id INT UNSIGNED NOT NULL,
FOREIGN KEY(attribute_id) REFERENCES attribute(id)
) Engine = InnoDB;
create entity_attributevalue join table
DROP TABLE IF EXISTS entity_attributevalue;
CREATE TABLE entity_attributevalue (
entity_id INT UNSIGNED NOT NULL,
attributevalue_id INT UNSIGNED NOT NULL,
FOREIGN KEY(entity_id) REFERENCES entity(id),
FOREIGN KEY(attributevalue_id) REFERENCES attributevalue(id)
) Engine = InnoDB;
populate entity table
INSERT INTO entity
(title, author, createdOn)
VALUES
('TestFile', 'Joe', '2011-01-01'),
('LongNovel', 'Mary', '2011-02-01'),
('ShortStory', 'Susan', '2011-03-01'),
('ProfitLoss', 'Bill', '2011-04-01'),
('MonthlyBudget', 'George', '2011-05-01'),
('Paper', 'Jane', '2012-04-01'),
('Essay', 'John', '2012-03-01'),
('Article', 'Dan', '2012-12-01');
populate attribute table
INSERT INTO attribute
(name, type)
VALUES
('ReadOnly', 'bool'),
('FileFormat', 'text'),
('Private', 'bool'),
('LastModified', 'date');
populate attributevalue table
INSERT INTO attributevalue
(value, attribute_id)
VALUES
('true', '1'),
('xls', '2'),
('false', '3'),
('2011-10-03', '4'),
('true', '1'),
('json', '2'),
('true', '3'),
('2011-10-04', '4'),
('false', '1'),
('ascii', '2'),
('false', '3'),
('2011-10-01', '4'),
('false', '1'),
('text', '2'),
('true', '3'),
('2011-10-02', '4'),
('false', '1'),
('binary', '2'),
('false', '3'),
('2011-10-20', '4'),
('doc', '2'),
('false', '3'),
('2011-10-20', '4'),
('rtf', '2'),
('2011-10-20', '4');
populate entity_attributevalue table
INSERT INTO entity_attributevalue
(entity_id, attributevalue_id)
VALUES
('1', '1'),
('1', '2'),
('1', '3'),
('1', '4'),
('2', '5'),
('2', '6'),
('2', '7'),
('2', '8'),
('3', '9'),
('3', '10'),
('3', '11'),
('3', '12'),
('4', '13'),
('4', '14'),
('4', '15'),
('4', '16'),
('5', '17'),
('5', '18'),
('5', '19'),
('5', '20'),
('6', '21'),
('6', '22'),
('6', '23'),
('7', '24'),
('7', '25');
Showing all the records
SELECT *
FROM `entity` e
LEFT JOIN `entity_attributevalue` ea ON ea.entity_id = e.id
LEFT JOIN `attributevalue` av ON ea.attributevalue_id = av.id
LEFT JOIN `attribute` a ON av.attribute_id = a.id;
id title author createdOn entity_id attributevalue_id id value attribute_id id name type 1 TestFile Joe 2011-01-01 00:00:00 1 1 1 true 1 1 ReadOnly bool 1 TestFile Joe 2011-01-01 00:00:00 1 2 2 xls 2 2 FileFormat text 1 TestFile Joe 2011-01-01 00:00:00 1 3 3 false 3 3 Private bool 1 TestFile Joe 2011-01-01 00:00:00 1 4 4 2011-10-03 4 4 LastModified date 2 LongNovel Mary 2011-02-01 00:00:00 2 5 5 true 1 1 ReadOnly bool 2 LongNovel Mary 2011-02-01 00:00:00 2 6 6 json 2 2 FileFormat text 2 LongNovel Mary 2011-02-01 00:00:00 2 7 7 true 3 3 Private bool 2 LongNovel Mary 2011-02-01 00:00:00 2 8 8 2011-10-04 4 4 LastModified date 3 ShortStory Susan 2011-03-01 00:00:00 3 9 9 false 1 1 ReadOnly bool 3 ShortStory Susan 2011-03-01 00:00:00 3 10 10 ascii 2 2 FileFormat text 3 ShortStory Susan 2011-03-01 00:00:00 3 11 11 false 3 3 Private bool 3 ShortStory Susan 2011-03-01 00:00:00 3 12 12 2011-10-01 4 4 LastModified date 4 ProfitLoss Bill 2011-04-01 00:00:00 4 13 13 false 1 1 ReadOnly bool 4 ProfitLoss Bill 2011-04-01 00:00:00 4 14 14 text 2 2 FileFormat text 4 ProfitLoss Bill 2011-04-01 00:00:00 4 15 15 true 3 3 Private bool 4 ProfitLoss Bill 2011-04-01 00:00:00 4 16 16 2011-10-02 4 4 LastModified date 5 MonthlyBudget George 2011-05-01 00:00:00 5 17 17 false 1 1 ReadOnly bool 5 MonthlyBudget George 2011-05-01 00:00:00 5 18 18 binary 2 2 FileFormat text 5 MonthlyBudget George 2011-05-01 00:00:00 5 19 19 false 3 3 Private bool 5 MonthlyBudget George 2011-05-01 00:00:00 5 20 20 2011-10-20 4 4 LastModified date 6 Paper Jane 2012-04-01 00:00:00 6 21 21 binary 2 2 FileFormat text 6 Paper Jane 2012-04-01 00:00:00 6 22 22 false 3 3 Private bool 6 Paper Jane 2012-04-01 00:00:00 6 23 23 2011-10-20 4 4 LastModified date 7 Essay John 2012-03-01 00:00:00 7 24 24 binary 2 2 FileFormat text 7 Essay John 2012-03-01 00:00:00 7 25 25 2011-10-20 4 4 LastModified date 8 Article Dan 2012-12-01 00:00:00 NULL NULL NULL NULL NULL NULL NULL NULL
pivot table
SELECT e.*,
MAX( IF(a.name = 'ReadOnly', av.value, NULL) ) as 'ReadOnly',
MAX( IF(a.name = 'FileFormat', av.value, NULL) ) as 'FileFormat',
MAX( IF(a.name = 'Private', av.value, NULL) ) as 'Private',
MAX( IF(a.name = 'LastModified', av.value, NULL) ) as 'LastModified'
FROM `entity` e
LEFT JOIN `entity_attributevalue` ea ON ea.entity_id = e.id
LEFT JOIN `attributevalue` av ON ea.attributevalue_id = av.id
LEFT JOIN `attribute` a ON av.attribute_id = a.id
GROUP BY e.id;
id title author createdOn ReadOnly FileFormat Private LastModified 1 TestFile Joe 2011-01-01 00:00:00 true xls false 2011-10-03 2 LongNovel Mary 2011-02-01 00:00:00 true json true 2011-10-04 3 ShortStory Susan 2011-03-01 00:00:00 false ascii false 2011-10-01 4 ProfitLoss Bill 2011-04-01 00:00:00 false text true 2011-10-02 5 MonthlyBudget George 2011-05-01 00:00:00 false binary false 2011-10-20 6 Paper Jane 2012-04-01 00:00:00 NULL binary false 2011-10-20 7 Essay John 2012-03-01 00:00:00 NULL binary NULL 2011-10-20 8 Article Dan 2012-12-01 00:00:00 NULL NULL NULL NULL
回答6:
However there are solutions to use lines as columns, aka transpose the data. It involve query tricks to do it in pure SQL, or you will have to rely on certain features only avaible in certain database, using Pivot tables (or Cross tables).
As exemple you can see how to do this here in Oracle (11g).
The programming version will be simplier to maintain and to make and moreover will work with any database.
回答7:
Partial answer since I do not know MySQL (well). In MSSQL I would look at Pivot tables or would create a temporary table in a stored procedure. It may well be a hard time ...
来源：https://stackoverflow.com/questions/649802/how-to-pivot-a-mysql-entity-attribute-value-schema